In an MRI system or MR scanner, an examination object, usually a patient, is exposed to a uniform main magnetic field (B0 field) so that the magnetic moments of the nuclei within the examination object tend to rotate around the axis of the applied B0 field (Larmor precession) with a certain net magnetization of all nuclei parallel to the B0 field. The rate of precession is called Larmor frequency which is dependent on the specific physical characteristics of the involved nuclei, namely their gyromagnetic ratio, and the strength of the applied B0 field. The gyromagnetic ratio is the ratio between the magnetic moment and the spin of a nucleus.
By transmitting an RF excitation pulse (B1 field) which is orthogonal to the B0 field, generated by means of an RF transmit antenna, and matching the Larmor frequency of the nuclei of interest, the spins of the nuclei are excited and brought into phase, and a deflection of their net magnetization from the direction of the B0 field is obtained, so that a transversal component in relation to the longitudinal component of the net magnetization is generated.
After termination of the RF excitation pulse, the relaxation processes of the longitudinal and transversal components of the net magnetization begin, until the net magnetization has returned to its equilibrium state. MR relaxation signals which are emitted by the transversal relaxation process, are detected by means of an MR/RF receive antenna.
The received MR relaxation signals which are time-based amplitude signals, are Fourier transformed to frequency-based MR spectrum signals and processed for generating an MR image of the nuclei of interest within an examination object. In order to obtain a spatial selection of a slice or volume within the examination object and a spatial encoding of the received MR relaxation signals emanating from a slice or volume of interest, gradient magnetic fields are superimposed on the B0 field, having the same direction as the B0 field, but having gradients in the orthogonal x-, y- and z-directions. Due to the fact that the Larmor frequency is dependent on the strength of the magnetic field which is imposed on the nuclei, the Larmor frequency of the nuclei accordingly decreases along and with the decreasing gradient (and vice versa) of the total, superimposed B0 field, so that by appropriately tuning the frequency of the transmitted RF excitation pulse (and by accordingly tuning the resonance frequency of the MR/RF receive antenna), and by accordingly controlling the gradient magnetic fields, a selection of nuclei within a slice at a certain location along each gradient in the x-, y- and z-direction, and by this, in total, within a certain voxel of the object can be obtained.
For generating the gradient magnetic fields, a gradient magnet system comprising a number of gradient magnets in the form of coils (gradient coils) is provided which is typically operated by means of a gradient amplifier system for generating electrical currents for supplying the gradient coils. Usually, such gradient coil currents have a certain waveform which has to be produced by the gradient amplifier system very precisely. The waveform is for example a trapezoid pulse with a pulse time of for example about 40 ms, a rise and fall time of the pulses of for example each about 0.2 ms, and having an amplitude in the order of several hundreds and up to about 1000 A. These current pulses have to be accurately controlled with a deviation of only several mA or less in order to ensure a generation of the MRI images at a high quality and high spatial resolution and precision.
WO 2010/004492 discloses a digital amplifier with feedforward and feedback control for generating and controlling the electrical output power for operating such a gradient magnet system. Basically, such a digital amplifier comprises an input for receiving a digital input signal representing the desired shape of a current curve of an analog output signal of the amplifier by which analog output signal the gradient magnet system is driven. The amplifier comprises a feedforward and a feedback controller in order to determine and compensate an error signal between the input signal and the output signal. The feedforward controller reads the input signal and predicts the output signal as accurate as possible based on a model of the system. Then the predicted output signal is subtracted in the analog domain from the measured analog output signal, and the resulting analog power differential current is converted into the digital domain for providing a feedback signal which is then combined with the feedforward signal. This results in a digital representation of the determined output signal which is now subtracted from the desired digital input signal so that an error signal results which is fed to a digital controller for providing an appropriate control signal to a modulator to counter the error signal. The modulator converts the control signal into a PWM signal which is provided to a power converter which generates the analog output signal for driving the gradient magnet system.